1. Field of the Invention
The present invention relates to an image printing apparatus for printing image data in at least two different types of printing forms, such as printing densities and printing colors.
2. Description of the Related Art
In making a document, one may desire to form printed images having different properties on a paper sheet. An image printing apparatus for printing characters, pictures and machine-readable codes corresponding to the characters and pictures has been proposed in Japanese Laid-Open Patent Publication No. 3-149682. Hereinafter, a machine-readable code is simply referred to as a "code". Such a conventional image printing apparatus combines input image data corresponding to characters and pictures with input image data corresponding to codes so as to output the combined image data to a printing control circuit. Then, the characters, picture, and codes are printed in the same form by the use of a single printing mechanism.
Actually, if a printing density of the image printing apparatus is 12 dots/mm, it is sufficient for printing characters and pictures for practical use. However, a higher printing density such as 16 dots/mm, 24 dots/mm or more is desired in order to print codes. This is because a printing area for codes can be reduced as the printing density increases.
Conventionally, characters, pictures and codes are printed in the same printing density. Accordingly, a printing density suitable for printing characters and pictures would not be sufficient for printing codes. On the contrary, a printing density suitable for printing codes would be too high for printing characters and pictures. This causes a problem of increase in the capacity of memories for storing printing data, increase in the scale of the circuit, decrease in the processing speed, and decrease in the printing speed.
Furthermore, in printing characters, the size of each printed dot is designed to be larger than the sub-scanning pitch so that a printed dot of a character corresponding to a line may overlap with another printed dot of a character corresponding to an adjacent line. This is because, in printing characters, it may be necessary to print the characters with no space between adjacent lines and/or to print thin lines. In printing codes, however, any space between printed dots of a code is likely to be eliminated in the case where a printed dot of a code corresponding to a line may overlap with another printed dot of a code corresponding to an adjacent line, thus making it difficult to detect the information represented by printed dots of codes.